Investigation into the impact of CeO2 morphology regulation on the oxidation process of dichloromethane

Four distinct CeO2 catalysts featuring varied morphologies (nanorods, nanocubes, nanoparticles, and nano spindle-shaped) were synthesized through a hydrothermal process and subsequently employed in the oxidation of dichloromethane (DCM). The findings revealed that the nano spindle-shaped CeO2 exhibited exposure of crystal faces (111), demonstrating superior catalytic oxidation performance for DCM with a T90 of 337 °C and notably excellent low-temperature catalytic activity (T50 = 192 °C). The primary reaction products were identified as HCl and CO2. Through obvious characterizations, it showed that the excellent catalytic activity presented by CeO2-s catalyst might be related to the higher oxygen vacancy concentration, surface active oxygen content, and superior redox performance caused by specific exposed crystal planes. Meanwhile, CeO2-s catalyst owned outstanding stability, reusability, and water inactivation regeneration, which had tremendous potential in practical treatment.


Introduction
Volatile Organic Compounds (VOCs) possess a multifaceted composition, undergoing decomposition upon exposure to light, thereby generating free radicals and peroxyl radicals.These radicals serve as pivotal precursors for the production of ozone and ne particulate matter, posing a substantial threat to the environment. 1Concurrently, VOCs are known to inict severe damage on the human respiratory, nervous, and immune systems. 2,3Notably, chlorinated volatile organic compounds (CVOCs) are characterized by high toxicity, volatility, and resistance to degradation, emerging as signicant pollutants with detrimental impacts on ecological ecosystems and human health. 4DCM, a representative CVOCs, nds widespread application as an organic solvent in sectors such as pharmaceuticals, spray coating, and rubber manufacturing. 5Urgency surrounds the imperative to fortify DCM treatment strategies.Common approaches for conducting the treatment of CVOCs encompass adsorption, absorption, condensation, combustion, low-temperature plasma, and catalytic oxidation. 6,7Catalytic oxidation, propelled by a catalyst, efficiently and comprehensively converts CVOCs into relatively non-toxic substances, including H 2 O, CO 2 , and HCl, with minimal energy consumption.It has evolved into the predominant technology applied in the CVOCs treatment sector in both China and internationally. 8Consequently, the preparation of catalysts that are efficient, stable, and cost-effective assumes paramount signicance in enhancing the competitiveness of this technology.While precious metal catalysts boast merits such as a low ignition temperature, high activity, and elevated HCl selectivity, their scarcity contributes to prohibitively high costs.Furthermore, the vulnerability of catalyst surfaces composed of precious metals to carbon deposition, coupled with the adsorption of chlorine from CVOCs onto active sites, leads to the phenomenon of chlorine poisoning, ultimately culminating in catalyst deactivation subsequent to chlorine deposition. 9,10In recent years, non-precious metal catalysts have garnered extensive scholarly attention in China and globally due to their relatively high activity, cost-effectiveness, and resilience against chlorine poisoning.
In addition, nanomaterials have unique physical, chemical and biological properties that make them promising for a wide range of applications in catalysis.Atul S. Nagpure et al. 11 studied the catalytic transfer hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) and furfural to 2-methylfuran (MF) using 2-propanol as hydrogen source on nitrogendoped mesoporous carbon (NMCs) supported Ru, Pd and Au metal catalysts.It was shown that highly dispersed Ru nanoparticles loaded on NMC exhibited excellent catalytic performance for the conversion of HMF to DMF and furfural to MF in the CTH reaction.This is mainly attributed to the smaller nanoparticle size of Ru (1.9 nm) and the good interaction between the metal and the carrier.Zhu et al. 12 used Cs 3 Sb 2 Br 9 perovskite nanoparticles (NPs) as a lead-free photocatalysts for photocontrolled atom transfer radical polymerisation (ATRP).Cs 3 Sb 2 Br 9 NPs have a high reduction potential, which enables efficient photo-induced reduction and controlls polymerisation of the initiator under blue light irradiation.Meanwhile, the Cs 3 Sb 2 Br 9 NPs can be recycled four times, showing good reusability.
China boasts abundant reserves of rare earth elements, establishing its position as the leading global rare earth supplier, contributing to over 90 percent of the world's total rare earth production annually.These rare earth elements nd extensive utilization in various applications, encompassing magnetic materials, luminescent materials, and catalysts dedicated to environmental protection. 13Among the rare earth materials, cerium dioxide (CeO 2 ) assumes a pivotal role due to its inherent attributes, including a stable cubic uorite structure.Notably, the facile interconversion between Ce 3+ and Ce 4+ ions endows CeO 2 with remarkable oxygen storage and release capabilities and a pronounced redox performance.Concurrently, this interconversion results in the creation of oxygen vacancies within the original lattice.Oxygen vacancies serve a dual function: rstly, they possess the capacity to adsorb gaseous-phase oxygen and transform it into surface-active oxygen species, and secondly, they serve as active sites for the direct adsorption of chlorinated volatile organic compounds (CVOCs), thereby enhancing catalytic efficiency.5][16] Furthermore, in addition to the intrinsic properties of CeO 2 delineated previously, the manipulation of CeO 2 's morphology exerts a signicant impact on both the selectivity towards exposed crystal faces and its redox performance capacity.This, in turn, exerts a profound impact on the catalytic oxidation performance.
Tian et al. 10 employed a one-step hydrothermal method to meticulously regulate the morphology of CeO 2 , specically targeting the exposure of distinct crystal faces.Their investigation revealed that, in comparison to alternative CeO 2 morphologies, nano-spherical CeO 2 , exposing the (111) crystal face, exhibited exceptional redox properties, lattice oxygen mobility, and emerged as the optimal catalyst for dichloroethane catalytic oxidation.In a similar vein, Hu et al. 17 manipulated CeO 2 morphology to modulate its selectivity towards exposed crystal faces.Their ndings indicated that rod-shaped CeO 2 , selectively exposing (110) and (100) crystal faces, displayed heightened mobility of reactive oxygen species, lower energy for oxygen vacancy generation, and superior catalytic oxidation performance for propane when contrasted with other morphologies.In light of these observations, this study endeavors to explore the modulation of CeO 2 selectivity towards exposed crystal faces through the manipulation of its morphology, thereby enhancing its catalytic efficacy on pollutants.Leveraging the hydrothermal synthesis method, four distinct nanomorphologies of CeO 2 were prepared, with DCM, an industrially prevalent compound, chosen as the subject of investigation.Conspicuously, nano spindle-shaped CeO 2 (CeO 2 -s) has not been used for catalytic oxidation of DCM.This paper meticulously examines and analyzes the impact of varied selectivities of CeO 2 , attributed to different nanomorphologies exposing crystal faces, on DCM catalytic performance, product selectivity, stability, reusability, and water resistance.Simultaneously, the catalyst undergoes rigorous physical and chemical characterization, encompassing morphology, crystal structure, specic surface area, average pore size, surface element valence and content, oxygen vacancy concentration, and redox performance.The synthesis of this comprehensive analysis constitutes an initial step in laying the foundation for the design and advancement of efficient catalysts specically craed for the mitigation of Chlorinated Volatile Organic Compound (CVOC) pollutants.

Catalyst preparation
The synthesis of catalyst is shown in Fig. 1.The synthesis of nano rod-shaped CeO 2 (CeO 2 -r) involved the following sequential steps: 2.17 g of Ce(NO 3 ) 3 $6H 2 O solid was dissolved in 20 mL of deionized water, followed by the gradual addition of a preprepared 60 mL, 6 mol L −1 NaOH solution to the Ce(NO 3 ) 3 -$6H 2 O solution using a rubber-tipped dropper.The resulting mixture was stirred at room temperature on a magnetic stirrer for 0.5 h and subsequently transferred to a 100 mL Teon highpressure hydrothermal autoclave for hydrothermal synthesis at 125 °C for 24 h.The synthesized product was then ltered, dried, and subjected to a nal roasting step in a muffle furnace at 500 °C for 3 h, yielding pale yellow rod-like CeO 2 powders.For the preparation of nano cube-shaped CeO 2 (CeO 2 -c), the same procedure as CeO 2 -r was followed, with the exception that the hydrothermal synthesis temperature was adjusted to 185 °C.The synthesis of nano particle-shaped CeO 2 (CeO 2 -p) involved the addition of 5 mmol of Ce(NO 3 ) 3 $6H 2 O and 40 mmol of urea to 80 mL of deionized water.Aer thorough mixing, the solution was placed in a 100 mL Teon high-pressure hydrothermal autoclave for hydrothermal synthesis at 180 °C for 10 h.Following natural cooling to room temperature, the product was ltered, dried, and subjected to a nal roasting step at 500 °C for 3 h to obtain particle-shaped CeO 2 powders.The preparation of nano spindle-shaped CeO 2 (CeO 2 -s) commenced with the addition of 2.4 mmol of Ce(NO 3 ) 3 $6H 2 O to 80 mL of deionized water in a microwave ultrasound instrument.Simultaneously, 6.4 mmol of urea was swily introduced to the cerium-containing solution and subjected to ultrasound concussion for 0.5 h.The resulting mixture was transferred to a magnetic stirrer, stirred at room temperature for 0.5 h, and then placed in a 100 mL Teon high-pressure hydrothermal autoclave at 130 °C for hydrothermal synthesis for 8 h.Aer cooling to room temperature, the product was centrifuged, ltered, and dried.The precursor powder was further calcined at 500 °C in a muffle furnace for 3 h to obtain spindle-shaped CeO 2 powders.
In summary, the total four different morphologies of CeO 2 need to be heated up from room temperature to 500 °C in a muffle furnace at a rate of 2 °C min −1 , and roasted at this temperature for 3 h.In this study, the different morphologies of CeO 2 were prepared by hydrothermal synthesis, and the morphology of CeO 2 was regulated by changing the hydrothermal temperature and time as well as the alkali concentration.Torrente-Murciano et al. 18 conducted a careful study of the conditions for the formation of cerium oxide nanomorphology and found that both the temperature and the concentration of alkali had a signicant effect on the CeO 2 morphology.In particular, lower hydrothermal temperatures favoured the synthesis of nanoparticulate CeO 2 , while higher temperatures made it easier to synthesize nanocubic CeO 2 .Liao et al. 19 prepared rod-shaped CeO 2 by ultrasound-assisted hydrothermal method, and found that cerium precursor, alkali concentration, and ultrasound were the critical to the formation of CeO 2 nanorods.

Catalyst characterization
Scanning electron microscopy (SEM) images of various CeO 2 morphologies were acquired using a SU-8020 Scanning Electron Microscope Operating at 30 kV.
Transmission electron microscopy (TEM) images, providing insights into the microscopic morphology and exposed crystal surfaces of the samples, were obtained using a JEM-2100HR Transmission Electron Microscope.
For the assessment of crystal structure, a Bruker D8 Advance X-ray powder diffractometer was employed.The analysis utilized a Cu Ka target with a wavelength (l) of 0.154058 nm, operating at 40 kV and 200 mA.The scanning parameters included a speed of 10°min −1 , and a scanning range spanning 2q = 10-80°.
The N 2 adsorption-desorption isotherm curve was generated at 77 K using the ASAP 2020 M automatic surface analyzer, a product of the US-based Micromeritics company.The specic surface area was determined employing the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was analyzed using the Barrett-Joyner-Halenda (BJH) method.
Photoelectron spectroscopy (XPS) of the CeO 2 samples was conducted utilizing the Thermo Scientic Escalab 250 Xi X-ray photoelectron spectrometer.The C 1s calibration binding energy was established at 284.8 eV.
Raman spectroscopic analysis (Raman) was executed employing the LabRAM Aramis Raman spectrometer from HYJ, France.The excitation light source had a wavelength of 325 nm (ultraviolet), and the scanning range spanned from 200 to 1400 cm −1 .
The temperature programmed reduction (H 2 -TPR) test was performed using the Auto Chem II 2920 chemisorption instrument.The sample underwent a temperature ramp from room temperature to 300 °C in a nitrogen (N 2 ) atmosphere (30 mL min −1 ) for 1 hour, followed by a return to room temperature.Subsequently, a 5% H 2 /Ar mixture was introduced as the reducing gas, and the sample temperature was elevated from room temperature to 800 °C at a heating rate of 10 °C min −1 .

Catalytic activity
The catalyst activity evaluation device is shown in Fig. 2. A cylindrical silica glass tube, possessing an inner diameter of 12 mm, served as the immobile reactor bed for assessing the catalytic oxidation efficiency of the prepared catalyst towards DCM.A quantity of 0.6 g of the catalyst sample (40-60 mesh) was carefully positioned within a quartz glass tube, affixed both above and below using an appropriate amount of passivated quartz wool.Subsequently, the reactor temperature underwent an incremental rise from room temperature to 150 °C, employing a heating rate of 5 °C min −1 , with a continuous ow of nitrogen set at 200 mL min −1 .The temperature was sustained for 0.5 h to mitigate the inuence of water vapor and other impurities on the experimental outcomes.A consistent concentration of DCM, incorporated into a gas mixture (20 vol% O 2 , N 2 as the equilibrium gas), was introduced into the system at a total gas ow rate of 200 mL min −1 .The tubular furnace initiated a programmed temperature ascent from 150 °C to 450 °C at a rate of 5 °C min −1 .Samples were extracted at pre-dened temperature intervals for subsequent analysis.The catalyzed gas underwent bifurcation for analysis.One portion, subsequent to condensation and desiccation, traversed through the CO infrared detector (SGA-700B-CO) and CO 2 infrared detector (SGA-700B-CO 2 ) to quantify concentrations of CO and CO 2 .Subsequently, it was directed towards the portable chlorine-containing gas detector to ascertain concentrations of HCl and Cl 2 .The remaining portion was introduced into the gas chromatography system (GC-7890A) to identify organic components within the reaction gas.The GC was equipped with an FID detector and an HP series capillary chromatographic column.
The calculations for DCM conversion rate and the yields of HCl, Cl 2 , CO 2 , and CO are delineated as follows:  The reaction rates for different morphologies of CeO 2 were compared and calculated as follows: wherein, C DCM denotes the initial concentration of DCM.V gas is the total ow rate of the reaction gas.m cat represents the mass of catalyst in the bed.X DCM is the conversion rate of DCM.

Morphology and microstructure of CeO 2 catalysts
The SEM was employed to scrutinize the morphological characteristics of the four CeO 2 catalysts.Fig. 3(a-f) distinctly illustrates randomly dispersed rods, angular cubes, centrally located particles, and conspicuous spindle-shaped CeO 2 , respectively, aligning seamlessly with the envisioned design.Simultaneously, all catalyst samples exhibited a uniformly homogeneous morphology within a specied eld of view, underscoring the consistent morphological integrity.In Fig. 3(a), CeO 2 -r was comprised of multiple nanorods with lengths of 40-350 nm.This morphology reveals inherent grain size heterogeneity, and a vertical observation of the nanorods indicates a solid structural conguration.Fig. 3(b and c), CeO 2 -c comprises numerous nanocubes, with ranging from 25-155 nm of side lengths.The smooth and angular surfaces of these nanocubes denote a high degree of crystallinity and a well-dened crystal structure. 20Fig. 3(d) illustrates CeO 2 -p, characterized by individual spherical nanoparticles with diameters falling within the range of 60-100 nm.The distribution of these nanoparticles appears more concentrated, with slight agglomeration.Moving on to Fig. 3(e), CeO 2 -s is depicted with multiple nanospindles, with signicantly larger size compared to other morphologies.These spindles exhibit lengths ranging from 4.5-10 mm and widths ranging from 1-2 mm.Upon closer examination of a single nanospindle at relatively high resolution, as depicted in Fig. 3(f), it becomes evident that the two ends of the nanospindles are relatively sharp, with curved edges differing from the smooth surface of the nanocube.Moreover, the surface of the nanospindle reveals a profusion of slits, indicating the presence of abundant defective sites.The nanospindle comprises several closely stacked nano-strips, affirming a non-monocrystalline structure.This tightly stacked framework is prone to inducing planar staggering and generating structural defects. 21s depicted in Fig. 4(a, c, e and g), the rod-shaped, cubic, particle, and spindle-shaped morphologies of CeO 2 are distinctly evident.Notably, each morphology exclusively appears within the visible range, providing additional evidence of the uniformity in morphology across all catalyst samples.Furthermore, the size results of the four distinct CeO 2 catalysts align consistently with the SEM ndings.For nano rod-shaped CeO 2 , the lattice fringe spacing predominantly measures 0.192 nm, corresponding to the (110) exposed crystal faces (Fig. 4(b)).Similarly, nano cube-shaped CeO 2 exhibits lattice spacing primarily at 0.271 nm, corresponding to the (100) exposed crystal faces of CeO 2 (Fig. 4(d)).The lattice spacing of nanoparticle CeO 2 is 0.313 nm, aligning with the (111) exposed crystal face (Fig. 4(f)). 22,23n the case of nano spindle-shaped CeO 2 , the structure comprises densely packed nanocrystals, indicative of a polycrystalline nature.Observations reveal several light spots on the crystal surface, suggesting the presence of defects.The lattice spacing of 0.313 nm corresponds to the (111) exposed crystal faces of CeO 2 (Fig. 4(h)).Signicant disparities in crystal size and specic exposed crystal faces are evident among CeO 2 morphologies. 23By modulating the growth of CeO 2 along different crystal faces, the selectivity of CeO 2 towards exposed crystal faces can be adjusted.This provides a basis for further exploration into the impact of CeO 2 catalysts with varying morphologies on the catalytic oxidation of DCM.
The crystal structures of the four distinct CeO 2 catalyst morphologies were meticulously examined through XRD mapping, as illustrated in Fig. 5. Characteristic diffraction peaks manifest at 2q angles of 28.5°, 33.1°, 47.4°, 56.4°, 59.0°, 69.5°, 76.9°, and 79.1°for CeO 2 catalysts with different morphologies.Comparative analysis with the standard card XRD (JCPDS PDF#34-0394) reveals that these diffraction peaks align with (111), ( 200), ( 220), (311), ( 222), (400), (331), and (420) exposed crystal faces, respectively.This unequivocally conrms that the four distinct CeO 2 morphologies prepared exhibit a typical cubic uorite structure with a space group of Fm3m and a cell parameter of a = 5.411 Å. 24 Both the intensity and width of diffraction peaks are intimately linked to the material's degree of crystallinity.Peak intensity correlates positively with crystallinity, while peak width correlates negatively. 25Analyzing characteristic diffraction peak intensities and peak halfmaximum full widths for all catalyst samples reveals a specic order: CeO 2 -c > CeO 2 -p > CeO 2 -r > CeO 2 -s for peak intensity, and CeO 2 -s > CeO 2 -r > CeO 2 -p > CeO 2 -c for peak width.Notably, CeO 2 -s exhibits the lowest diffraction peak intensity and the highest peak width among the various CeO 2 morphologies, signifying relatively low crystallinity and a propensity for lattice defects.This nding aligns consistently with the SEM and TEM results.By applying Scherrer's formula to the full width at half maximum of the diffraction peak at 2q = 28.5°, the grain sizes of CeO 2 with different morphologies were calculated.The resulting order of grain sizes is CeO 2 -c (26.47 nm) > CeO 2 -p (22.08 nm) > CeO 2 -r (12.06 nm) > CeO 2 -s (8.82 nm), with spindle-shaped CeO 2 boasting the smallest grain size.The grain size of CeO 2 signicantly inuences the content of surface oxygen species and the concentration of defective oxygen vacancies. 26Smaller grain size CeO 2 catalysts are advantageous for exposing surfaceactive sites, thereby exhibiting heightened catalytic degradation effects.
The N 2 adsorption-desorption isotherms and BJH pore size distribution for CeO 2 with various morphologies are elucidated in Fig. 6.The adsorption-desorption isotherms of the four CeO 2 catalyst morphologies showcase typical H3-type hysteresis loops within the P/P 0 range of 0.4 to 0.97.Following IUPAC classication, the isotherms for all CeO 2 catalysts, regardless of morphology, fall under type IV, signifying narrow mesoporous structures for each. 27The starting height of the hysteresis loop is proportionate to the specic surface area of the samples. 28otably, the loop starting heights follow the order: CeO 2 -s > CeO 2 -r > CeO 2 -p > CeO 2 -c.This implies that CeO 2 -s boasts the largest specic surface area, aligning consistently with the ndings presented in Table 1.Concurrently, the pore size distribution (BJH) of CeO 2 catalysts is depicted in the gure, revealing varying but mesoporous structures for all   morphologies.Specic surface area, pore volume, and pore size for the four CeO 2 catalysts are detailed in Table 1 Notably, the specic surface area of CeO 2 -s stands at 107.8 m 2 g −1 , signicantly surpassing CeO 2 -r (85.9 m 2 g −1 ), CeO 2 -c (40.5 m 2 g −1 ), and CeO 2 -p (70.6 m 2 g −1 ).Tamboli et al. 29 similarly prepared four different shapes of CeO 2 , namely sphere, mixed shape, spindles, and rod, where CeO 2 spindles exhibited the largest specic surface area (104 m 2 g −1 ), which is consistent with the results tested here.A larger specic surface area facilitates increased exposure of active sites, thereby enhancing pollutant adsorption on the catalyst surface and promoting more thorough oxidative decomposition. 30Pore size analysis indicates similar sizes for CeO 2 -r (20.7 m 2 g −1 ), CeO 2 -c (23.2 nm), and CeO 2 -p (27.9 nm) catalysts, while CeO 2 -s exhibits a markedly smaller pore size of 3.4 nm.A smaller pore size suggests poorer crystallization effects, rendering the material prone to lattice defects and subsequently elevating the surface concentration of oxygen vacancies.This observation aligns with the combined analysis of SEM and XRD characterizations.

Surface chemical states
X-ray Photoelectron Spectroscopy (XPS) serves to elucidate the electronic layering of atoms or molecules present on the catalyst surface, facilitating a comprehensive analysis of surface elemental composition and valence states within the catalyst specimen.In Fig. 7(a), the Ce 3d photoelectron spectra of CeO 2 catalysts with diverse morphologies are depicted.It is noteworthy, as elucidated in the pertinent literature, 31 that the deconvolution process resulted in the identication of eight distinct sets of peaks for Ce 3d, encompassing two spin orbitals, namely 3d 5/2 (u) and 3d 3/2 (v).This nding signies the coexistence of two valence states, Ce 4+ and Ce 3+ , on the surfaces of CeO 2 catalysts with varying morphologies.
The distinctive peaks in the Ce 3d spectrum, denoted as u (900.8 eV), u 00 (907.2 eV), u 00 (816.6 eV), v (882.3 eV), v 00 (888.6 eV), and v 00 (897.7 eV), are unequivocally associated with Ce 4+ .In contrast, the characteristic peaks located at u 0 (902.5 eV) and v 0 (884.9eV) correspond to Ce 3+ .This phenomenon entails the relinquishment of electrons by lattice oxygen atoms, leading to their departure from the lattice site and subsequently giving rise to the generation of oxygen vacancies.
Simultaneously, Ce 4+ undergoes electron acquisition, converting into Ce 3+ , thereby signifying the occurrence of oxygen vacancy generation.The presence of Ce 3+ serves as an indicator, and its concentration exhibits a positive correlation with the oxygen vacancy concentration. 32Oxygen vacancies on the catalyst surface facilitate the adsorption of gas-phase oxygen, and a sequential migration process occurs as follows: O 2 / 2O / O 2 − / 2O − / 2O 2− . 33This migration mechanism promotes the catalytic oxidation of DCM.To semi-quantitatively assess the ratio of Ce 3+ to total Ce in different orbitals, calculations were based on the characteristic peak area ratio of Ce 3+ to total Ce.As outlined in Table 2, CeO 2 -s exhibits the highest Ce 3+ content across various orbitals.This observation may be attributed to the nano spindle-shaped CeO 2 , as revealed in transmission electron microscopy (TEM), featuring a concaveconvex interface resulting from extensive planar interlacing.The elevated Ce 3+ content indicates a higher concentration of oxygen vacancies in this morphology, which, in turn, adsorb and activate gas-phase oxygen, transforming into highly active surface oxygen species.This process promotes their migration, thereby enhancing the catalytic oxidation performance of DCM.Conversely, CeO 2 -c demonstrates a lower Ce 3+ content compared to the other three morphologies.This observation aligns with the crystallographic analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD), suggesting  , serving as active oxygen species generated on oxygen vacancies, effectively catalyzing the degradation of adsorbed DCM.Consequently, the ratio of O ads /(O latt + O ads ) serves as an indicator of O ads concentration and provides an assessment of oxygen vacancy concentration. 36Quantitative analysis of characteristic peak areas, as detailed in Table 2, reveals the following order for the O ads /(O latt + O ads ) ratio: CeO 2 -s (35.18%) > CeO 2 -p (32.31%) > CeO 2 -r (31.63%) > CeO 2 -c (28.71%).CeO 2 -s exhibits a notably higher ratio, indicating a more substantial concentration of surface adsorbed oxygen compared to other CeO 2 morphologies.This nding further substantiates that the nano spindleshaped CeO 2 surface features the highest oxygen vacancy concentration, a conclusion consistent with the Ce 3d spectroscopy results.
The molecular structure of the prepared catalysts was scrutinized through Raman spectroscopy, and the outcomes of Raman spectroscopic characterization for CeO 2 with distinct morphologies are presented in Fig. 8 All four diverse nanomorphologies of CeO 2 catalysts exhibit prominent characteristic peaks at 460, 595, and 1182 cm −1 .The Raman peak observed at 460 cm −1 corresponds to the symmetric telescopic vibrational peaks (F 2g ) associated with the Ce 4+ cation and the surrounding eight O 2− anions in the cubic uorite structure of CeO 2 .This observation corroborates the cubic crystalline structure of CeO 2 , aligning with the X-ray diffraction (XRD) results. 37The Raman peak at 595 cm −1 is attributed to the characteristic peak of oxygen vacancy, specically the Frenkel defect induction mode (D) induced by the presence of Ce 3+ . 38dditionally, the Raman peak at 1182 cm −1 corresponds to the second-order longitudinal optical vibration peak (2LO). 39In general, the ratio of peak D to peak F 2g intensity (I D /I F 2g ) allows for the quantitative analysis of the surface oxygen vacancy concentration in CeO 2 .A higher ratio indicates a greater surface oxygen vacancy concentration. 40,41Oxygen vacancies, as signicant structural defects in metal oxides, serve as a pivotal reference index for evaluating catalytic oxidation performance.They not only adsorb gas-phase oxygen molecules, forming highly reactive surface oxygen species in catalytic reactions, but also directly act as adsorption sites for pollutants, enhancing catalytic degradation effectiveness.The I D /I F 2g ratios for the four distinct nanomorphologies of CeO 2 catalysts follow the order: CeO 2 -s > CeO 2 -r > CeO 2 -p > CeO 2 -c.This sequence further affirms that the relatively high concentration of oxygen vacancies on the surface of nano spindle-shaped CeO 2 is conducive to the catalytic oxidation of DCM, a conclusion consistent with the X-ray photoelectron spectroscopy (XPS) results.

Temperature programmed characterizations
CeO 2 , owing to its facile interconversion between Ce 3+ and Ce 4+ , possesses exceptional redox properties and nds widespread applications in the eld of catalytic oxidation.The lowtemperature reduction properties of four distinct nanomorphologies of CeO 2 were investigated using H 2 -TPR spectroscopy.Typically, the reduction of CeO 2 can be categorized into three temperature intervals, namely surface oxygen reduction at 250-400 °C, subsurface oxygen reduction at 400-600 °C, and bulk oxygen reduction at 600-1000 °C. 42As depicted in Fig. 9, all four diverse nanomorphologies of CeO 2 exhibit two sets of high-resolution reduction peaks.The reduction peak at <600 °C corresponds to the reduction of CeO 2 surface oxygen, whereas the peak at >600 °C signies the reduction of CeO 2 bulk-phase oxygen. 43The primary focus centers on the investigation of the reduction peak in the low-temperature range (<600 °C), as it directly reects the extent of involvement of surface-  Paper RSC Advances active oxygen species and is intricately linked to the catalytic performance at low temperatures.Evident from the sequence of reduction peak signals, the temperatures at which the reduction peaks occur follow the order: CeO 2 -c > CeO 2 -p > CeO 2 -r > CeO 2 -s.CeO 2 -c exhibits a reduction peak at 485.4 °C, while the reduction peaks for other CeO 2 morphologies shi towards lower temperatures.Specically, CeO 2 -s registers the lowest peak temperature, with a maximum temperature difference of 64.4 °C compared to CeO 2 -c.Generally, a lower temperature for the occurrence of the reduction peak signies superior redox performance.5][46] This observation suggests that the surface oxygen species on the nano spindle-shaped CeO 2 catalyst are more active, demonstrating enhanced low-temperature reduction performance.To delve deeper into the reduction performance of the prepared catalyst, hydrogen consumption was analyzed.Based on the peak area of the hydrogen consumed by reduction, the chemisorbent for the test had the peak area of the corresponding hydrogen consumption calibrated, which led to the calculation of the hydrogen consumption during the TPR of different samples. 47,48s shown in Table 3, In the low-temperature interval, the hydrogen consumption of CeO 2 with different nanomorphologies follows the order: CeO 2 -s > CeO 2 -r > CeO 2 -p > CeO 2 -c, with the hydrogen consumption of CeO 2 -s being 2.13 times that of CeO 2 -c.It is well-established that hydrogen consumption is directly proportional to the content of surfaceactive oxygen species. 49,50The hydrogen consumption results further indicates that the spindle-shaped CeO 2 catalysts, featuring exposed crystal faces of (111) are abundant in surfaceactive oxygen species.Consequently, CeO 2 -s exhibit excellent low-temperature reduction properties and hold signicant potential for catalyzing the oxidation of DCM.Notably, this nding aligns with the X-ray photoelectron spectroscopy (XPS) results.

Evaluation of catalyst activity
The catalytic degradation effects on DCM by the four prepared catalysts with different nanomorphologies (CeO 2 -r, CeO 2 -c, CeO 2 -p, CeO 2 -s) in the temperature interval of 150-450 °C are illustrated in Fig. 10.The corresponding temperatures (T 50 , T 90 ), representing the achievement of 50% and 90% conversion rates of DCM over the different morphologies of CeO 2 catalysts, are presented in Table 4.As depicted in Fig. 10(a), the conversion rate of DCM over the four different nanomorphologies of CeO 2 catalysts exhibits an ascending trend with increasing  This temperature is signicantly lower than the other three morphologies, and when compared with CeO 2 -c, the difference between the two-DT 50 = 112 °C.This result suggests that the nano spindle-shaped CeO 2 exhibits superior catalytic activity at low temperatures.The order in which different nanomorphologies of CeO 2 reach T 90 is as follows: CeO 2 -c (399 °C) > CeO 2 -p (347 °C) > CeO 2 -r (344 °C) > CeO 2 -s (337 °C), with CeO 2 -s achieving T 90 at the lowest temperature, exhibiting a difference of DT 90 = 62 °C compared to CeO 2 -c.The nano spindle-shaped CeO 2 catalysts results show that featuring exposed crystal faces of (111) is signicantly robust catalytic degradation effects on DCM.Considering the aforementioned characterization results, the nano spindle-shaped CeO 2 catalysts, in comparison with the other three morphologies of CeO 2 catalysts, exhibit characteristics such as a large specic surface area, small pore size, weak crystallinity, robust low-temperature reduction performance, high oxygen vacancy concentration, and abundant surface oxygen species.These characterization results further affirm that nano spindle-shaped CeO 2 catalysts exert a more pronounced catalytic oxidation effect on DCM.
In Fig. 10(a), all four CeO 2 catalysts with different morphologies exhibit a transient deactivation phenomenon in the temperature interval of 200-250 °C.The conversion rates of DCM decrease to varying degrees with the rise in temperature, a behavior that may be attributed to the strong adsorption of chlorine (Cl) species from DCM at lower temperatures onto active sites on the catalyst surface. 51,52As the reaction temperature increases and oxygen diffuses through the surface lattice, the Cl species adsorbed on the catalyst surface gradually desorb, leading to a restoration of the activities of the various CeO 2 catalyst morphologies, with an increasing trend.Fig. 10(b) shows the reaction rates of CeO 2 catalysts with different morphologies at 150-400 °C, the order in which different nanomorphologies of reaction rates are as follows: CeO 2 -s > CeO 2 -r > CeO 2 -p > CeO 2 -c.Which indicates that CeO 2 -s has the highest reaction rate for the most efficient conversion of DCM as the temperature increases, and this result is in agreement with Fig. 10(a).

Product distribution of catalysts
The nature of a catalyst plays a crucial role in determining its product distribution.To investigate the correlation between exposed crystal faces and product selectivity, CeO 2 nanomorphologies were deliberately controlled by altering the preparation conditions.This regulation aimed to inuence the selectivity towards exposed crystal faces during the catalytic oxidation of DCM using different morphologies of CeO 2 .The catalytic products, including HCl, Cl 2 , CO 2 , and CO in the exhaust gas, were collected, and the yields were calculated and analyzed using the provided equation.The HCl yield of the catalytic products from the four distinct nanomorphologies of CeO 2 catalysts is illustrated in Fig. 11 The HCl yield of CeO 2 catalysts, across all samples, exhibits a gradual increase in the temperature interval of 150-250 °C.During this interval, the HCl yield for all catalysts remains below 10%, potentially attributed to the fact that HCl does not reach its desorption temperature before 250 °C.As the temperature rises to 300 °C, the incline of the HCl yield curves for all catalysts notably intensies.The distribution of Cl 2 yield for different morphologies of CeO 2 catalysts is depicted in Fig. 11(b).All catalysts generate a certain amount of Cl 2 at higher temperatures (>300 °C), a phenomenon that may be associated with the Deacon reaction (4HCl + O 2 / 2Cl 2 + 2H 2 O).CeO 2 -s exhibits a notably high Cl 2 yield.In conjunction with Xray photoelectron spectroscopy (XPS) and Raman characterization results, CeO 2 -s demonstrates a higher oxygen vacancy concentration than other morphologies.This characteristic promotes the migration of surface lattice oxygen, facilitating the diffusion of lattice oxygen to the outer surface of the catalyst.This process replaces Cl species on the oxygen vacancy, thereby contributing to the generation of Cl 2 .Additionally, the Cl 2 generation temperature of CeO 2 -s is 315 °C, lower than that of the other three morphologies.This is attributed to the richer content of surface oxygen species on the surface of the CeO 2 -s catalyst.The Cl 2 generation temperature of CeO 2 -p is observed to be 335 °C, indicating an increased temperature for Cl 2 generation.The yields of CO 2 and CO for all catalyst samples are depicted in Fig. 11(c and d), offering insight into the redox properties by comparing the CO x yield of CeO 2 with different morphologies.The CO x yield for all four nanomorphologies of CeO 2 increases with temperature, yet their yields differ.Specically, CeO 2 -s exhibits the highest CO 2 yield, CeO 2 -r and CeO 2 -p demonstrate similar yields, while the CO 2 yield of CeO 2c is notably lower than the three aforementioned morphologies.These results indicate that nano spindle-shaped CeO 2 catalysts with exposed crystal faces of (111) possess superior redox capacity.Furthermore, the CO yield of all catalysts was analyzed Fig. 11(d), revealing that CeO 2 -c has a higher CO content than the other three morphologies.This suggests that the lower redox capacity of cubic CeO 2 itself results in an inability to efficiently and swily oxidize intermediate transition products and CO generated during the reaction process to CO 2 , leading to a higher CO yield.

Stability and durability of catalysts
The long-term stability and reusability of CeO 2 -s in the catalytic oxidation of DCM were examined.The catalytic degradation rate of DCM by CeO 2 -s, operating continuously at a constant temperature of 340 °C for 48 hours, is illustrated in Fig. 12(a).The results indicate that there is no signicant activity loss of the catalyst during the experiment, and the degradation rate of DCM remains stable at about 90% until the end of the experiment.This outcome suggests that CeO 2 -s exhibits good chlorine resistance and resistance to carbon accumulation, demonstrating stability in the catalytic oxidation of DCM.Additionally, four consecutive cycle tests were conducted on CeO 2 -s, and the degradation effect of DCM within the temperature interval of 150-450 °C is presented in Fig. 12(b).Throughout the continuous cycle test, the catalytic activity of CeO 2 -s shows a slight decreasing trend, but this has minimal impact on the overall catalytic reaction.In the four cycles, CeO 2 -s achieves T 50 for DCM catalytic oxidation before 200 °C and T 90 before 350 °C.This outcome further emphasizes the good stability of nano spindle-shaped CeO 2 , making it suitable for reuse in practical catalytic DCM applications and contributing to reduced treatment costs.

Effect of WHSV
The impact of various airspeed conditions (Weight Hourly Space Velocity-WHSV) on the degradation of DCM over CeO 2 -s catalysts was investigated, as illustrated in Fig. 13.As the WHSV increases from 20 000 mL g −1 h −1 to 60 000 mL g −1 h −1 , a decreasing trend is observed in the activity of CeO 2 -s catalysts.However, catalysts at different WHSV levels demonstrate the ability to achieve T 90 at 350 °C against DCM, indicating enhanced resistance to DCM impact for nano spindle-shaped CeO 2 catalysts.Specically, at WHSV = 20 000 mL g −1 h −1 , T 50  and T 90 are 192 °C and 337 °C, respectively, marking a decrease of 73 °C and 11 °C compared to WHSV = 60 000 mL g −1 h −1 .These results suggest that at lower WHSV, DCM exhibits a prolonged residence time on the CeO 2 -s catalyst surface compared to higher WHSV conditions.This prolonged exposure facilitates the adsorption of DCM on the catalyst surface for activation, thereby promoting the catalytic degradation of DCM.

Water resistance of catalysts
CeO 2 -s catalysts demonstrate effectiveness in the catalytic oxidation of DCM under ideal drying conditions.However, typical industrial exhaust gases oen contain approximately 5 vol% moisture, a factor that commonly hampers catalyst activity and inuences the catalytic degradation efficiency. 53,54he water resistance of CeO 2 -s catalysts exhibiting optimal DCM catalytic oxidation performance was investigated by introducing 1 vol% and 5 vol% H 2 O into the mixed dry gas.As depicted in Fig. 14, at a constant temperature of 400 °C, the CeO 2 -s catalyst maintains stable activity with DCM conversion hovering around 99.5%.Upon the addition of 1 vol% H 2 O to the gas mixture, the CeO 2 -s catalyst experiences show a slight impact on DCM conversion, resulting in a decrease in DCM conversion to approximately 98%.Meanwhile, HCl production increased from 51.17% to 58.21% and Cl 2 production decreased from 25.32% to 21.27%.Upon cessation of water vapor addition to the gas mixture, DCM conversion and HCl/Cl 2 production gradually reverts to its initial level before reaching stability.Upon the addition of 5 vol% H 2 O to the gas mixture, the CeO 2 -s catalyst experiences a noticeable impact, resulting in a decrease in DCM conversion to approximately 93.29%.The results indicate that higher concentrations of water vapour cause slight catalyst deactivation.Despite the continuous introduction of a xed amount of water vapor, DCM conversion remains stable at around 93%, underscoring the water resistance of nano spindleshaped CeO 2 .Meanwhile, HCl production increased to 66.25% and Cl 2 production decreased to 18.12%.Upon cessation of water vapor addition to the gas mixture, DCM conversion and HCl/Cl 2 production gradually reverts to its initial level before reaching stability.The results indicate that water vapour in the gas mixture promotes the conversion and desorption of chlorine species adsorbed on the catalyst surface to HCl and inhibits the positive operation of the Deacon reaction, leading to an increase in the production of HCl and a decrease in the production of Cl 2 .At the same time, water molecules readily adsorb on the active sites of the catalyst surface, competing with DCM for adsorption.This competition renders the activation of DCM adsorption on the CeO 2 -s surface more challenging, consequently diminishing DCM conversion and relating with HCl/Cl 2 production.However, it's noteworthy that the inhibition of catalyst activity by water vapor is of a physical nature, and this inhibition dissipates upon the removal of water vapor.

Conclusions
In this work, four CeO 2 catalysts presented different catalytic performance, and the inuence of exposed crystal surface was accordingly investigated.Compared to CeO 2 -r, CeO 2 -c and CeO 2 -p catalysts, CeO 2 -s catalysts exhibited the best DCM catalytic oxidation (T 90 = 337 °C, T 50 = 192 °C) and the higher HCl and CO 2 production rates.Obvious characterizations included that CeO 2 -s catalysts owned smaller grain size, higher specic surface area, more surface oxygen species, higher concentration of oxygen vacancies, and low-temperature reductivity.Among them, smaller grain size and larger specic surface area contributed the most in the catalytic oxidation of DCM.Extensive performance tests were conducted on CeO 2 -s catalysts.Specically, stability and durability tests suggested that CeO 2 -s exhibited robust stability and maintained consistently high activity over four usage cycles, showcasing excellent reusability and cost-effectiveness in practical applications.Water resistance and varying airspeed effect tests demonstrated that CeO 2 -s displayed efficient activity recovery aer water vapor removal, while the catalytic activity for DCM performed a decreasing trend with increasing weight hourly space velocity.
It has been shown that the shape of CeO 2 nanoparticles is inuenced by multiple factors, including the nature of the solvent, the concentration of precursors, the use of additives, and the temperature and time of the reaction.The following are several factors that may inuence the growth mechanism of CeO 2 nanoparticles:

Solvent effect
The nature of the solvent (e.g.polarity, dielectric constant, etc.) can signicantly affect the nucleation and growth process of nanoparticles.Heterogeneous solvents have different effects on the solubility of CeO 2 precursors and the diffusion rate of reactants, thus changing the shape and size of particles.

Precursor concentration
The concentration of the precursor will affect the rate of nucleation and growth.Higher concentrations of precursors lead to rapid nucleation and the formation of smaller particles.The lower concentration may promote the growth of particles and form larger particles.

Additives
Additives or surfactants used in the synthesis process can be adsorbed onto specic crystalline surfaces, thereby affecting the direction and shape of particle growth.These molecules promote the formation of particles of specic shapes by selectively hindering the growth of certain crystalline surfaces.

Reaction temperature and time
Reaction temperature and time are crucial elements for the growth of CeO 2 nanoparticles.Higher reaction temperatures generally accelerate nucleation and growth processes, while longer reaction times allow for further particle growth, which together determine the nal shape and size of the particle.

Crystal growth kinetics
The growth of CeO 2 nanoparticles is also affected by crystal growth kinetics.Crystal growth anisotropy (i.e., differences in growth rates at different crystal planes) can lead to the formation of nanoparticles with diverse shapes.
wherein,[DCM] in and [DCM] out denote the DCM concentration at the reactor inlet and outlet, respectively.Similarly, [HCl] out signies the hydrogen chloride concentration at the outlet, [Cl 2 ] out represents the chlorine concentration at the outlet, [CO 2 ] out indicates the carbon dioxide concentration at the outlet, and [CO] out denotes the carbon monoxide concentration at the outlet.
(a).It is evident that all catalysts exhibit an increasing trend in HCl yield within the temperature interval of 150-450 °C.The yield follows the order: CeO 2 -s > CeO 2 -p > CeO 2 -r > CeO 2 -c.The prepared catalysts achieve yields exceeding 50%, indicating robust HCl selectivity.Evidently, the nano spindle-shaped CeO 2 catalysts with exposed crystal faces (111) demonstrate the highest HCl yield at 65.67%, underscoring their superior performance in this regard.

Fig. 12
Fig. 12 Stability performance test of CeO 2 -s catalyst for DCM catalytic oxidation (a), durability test of CeO 2 -s catalyst for DCM catalytic oxidation at 150-450 °C (b).

Fig. 13
Fig. 13 Effect of different WHSV on the DCM reaction performance of CeO 2 -s catalysts.

Fig. 14
Fig. 14 Effect of 1 vol% and 5 vol% H 2 O on DCM reaction performance over CeO 2 -s catalysts at 400 °C.

Table 1
Specific surface area, pore volume, pore size and grain size of CeO 2 catalysts with different nanomorphologiesSamplesS BET (m 2 g −1 )

Table 3
Hydrogen consumption of CeO 2 with different nanomorphologies .All catalyst samples demonstrate complete catalysis of DCM oxidation at temperatures up to 450 °C.However, during the programmed temperature increase (<450 °C), the catalytic oxidation effects of different nanomorphologies of CeO 2 on DCM vary signicantly.The order of reaching T 50 is as follows: CeO 2 -c (304 °C) > CeO 2 -p (277 °C) > CeO 2 -r (268 °C) > CeO 2 -s (192 °C), with CeO 2 -s achieving T 50 at less than 200 °C.
Fig. 10 DCM catalytic performance (a) and reaction rate (b) of CeO 2 with different nanomorphology.temperature

Table 4
Catalytic activity of CeO 2 catalysts with different nanomorphologies